A firm foundation is essential to good performance of buildings and other load-bearing structures. The foundation includes properly installed footings of adequate size to support a structure and prevent excessive settling. Foundation systems are classified as shallow and deep foundations, depending on the depth of the load-transfer member below the super-structure and the type of transfer load mechanism. The required foundation system depends on several factors or conditions such as the strength and compressibility of the site soils, the proposed loading conditions, and the project performance criteria (i.e. total settlement and differential settlement limitations.)
In construction sites where settlement is not a problem, shallow foundations provide the most economical systems. Shallow foundations are typically placed from ground level to 3 meters below ground level or below the frost line. Shallow foundation construction is typically utilized for most residential and light commercial raised floor building sites. FIG. 1, building structure 10 is built on shallow foundation 12. The shallow foundation may be of any suitable shape such as the inverted “T” shape shown. This shape allows more stability.
Where poor soil conditions are found, deep foundations may be needed to provide the required load-bearing capacity and to limit settlement. FIG. 1, building structure 14 is built on a deep foundation 16. Examples of deep foundation systems include driven piles (i.e. pressure-treated timber piles, concrete, or steel), drilled shafts, or micro piles.
Foundation specifications, including footing requirements, are covered in various building codes, and sized in accordance with the building capacity of the soil and the weight of the building. In areas subject to seasonal frost, the bottom of the footing must be placed below the frost line to prevent damage to the footing and structure due to frost heave.
A raised foundation is a foundation which is raised above the plane of the surrounding earth. The main floor of a home or business is built on this foundation. A post and pier foundation system is one example of a raised system. Poured concrete footings are often used in raised foundations. In one example, a wood, metal, plastic, or composite post is set in the ground with concrete and bears the weight of the structure on it. The post is below grade.
In another example, a concrete pier extends from the footing base to above grade. There are several variations of this footing type. FIG. 2 and FIG. 3 depict concrete footings that extend below the frost line to above the ground. Both footings have a wood post 20, typically a 6″×6″ post, attached above ground to the concrete pier. For example, in FIG. 2, an anchor bolt 22 is used to connect the post to a concrete footing 24. A gravel base 26 may be used at the bottom of the concrete footing to prevent frost heave. In FIG. 3, a concrete footing 30 is poured in fiber tube 32, a metal post anchor 34 is placed in the concrete, and then the concrete is allowed to set. The above ground portion of the fiber tube is removed after the concrete is set. The metal post anchor 34 connects the post 20 to the concrete footing. Such concrete footings typically extend six inches below the frost line 36 and rest on undisturbed soil 38. The top of the footing is typically at least 6″ above grade.
Wood posts are usually attached to the top of the concrete footing above ground. Untreated wood posts will quickly rot if placed below ground due to the presence of water and oxygen which results in fungal attack, for example. Likewise, untreated metal posts placed below ground will rust. Pressure treated wood is available for use in ground contact applications, some having warranties as long as 75 years, however they are expensive. Galvanized metals are used for underground applications. Because such foundations rely on anchors, the structure can be compromised if the anchor bolt becomes loose or breaks.
Concrete has many drawbacks. For instance, concrete takes time to cure, is heavy, porous, and brittle, has high labor costs, has a high carbon footprint, needs large quantities of water, and cannot be poured below 5° C. It is desirable to provide an alternative to concrete footing systems.
The use of polyurethane foams for setting posts has been previously taught, however, the prior art doesn't disclose the use of polyurethane foam for applications where the purpose of the foam is to increase the load bearing capacity of a foundation. U.S. Pat. Nos. 3,403,520 and 5,466,094 disclose the use of polyurethane as a foamable liquid for use in the installation of utility poles, where the pole bears the load and the foam is used to surround the pole and allow for the sole reduction in the size of a hole. While the present invention seeks to do the opposite by expanding the size of the hole, such that the load is spread over a larger area.